Is there some special combination of buttons to perform the new extended assassinations in Halo: Reach? Like where you knife someone? I can only seem to do they typical Halo behind the back assassination when pushing RB.

If you play the 3rd mission in the campaign (Nightfall, I think - the one where you start w/ a sniper rifle), it sets you up to assassinate an Elite early on, complete with a button prompt. That might help you to do it in multiplayer.
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AdeeseJan 3 '11 at 20:35

Yep you need to hold the button rather than press it - that's how the game decides which you do.

Assasinating someone is not always the best option, if they are weak and there are several enemies around it is best to just tap it for a regular beatdown as you can start attacking other enemies sooner. Also while you are assasinating someone you're an easy target for enemies.